A conventionally known electrophotographic method involves: utilizing a photoconductive substance to form an electrical latent image on a photosensitive member with the aid of various means; developing the latent image with toner; transferring the toner image onto a transfer material such as paper as required; and fixing the toner image under heat, pressure, or heat and pressure, or by means of the vapor of a solvent to produce a toner image.
A magnetic one-component development system using magnetic toner is preferably used for a method involving development with toner because the system eliminates the need for a carrier and is advantageous of a reduction in size of an apparatus. A considerable amount of magnetic body in fine powder form is mixed with and dispersed into toner to be used for the magnetic one-component development system. The state of presence of the magnetic body greatly affects the fluidity and triboelectric chargeability of the toner.
When one attempts to reduce the particle size of magnetic toner to improve dot reproducibility and the like, the amount of a magnetic body to be added has been conventionally increased to maintain a good balance between charging property and magnetic property. In this case, however, the following problems occur. One problem is that the amount of a binder component in the toner is relatively reduced to inhibit low-temperature fixability. Another problem is that fluidity is apt to reduce in association with increases in the saturated magnetization and true specific gravity of the toner and napping in an aggregated state is apt to be formed on a toner carrier, so it becomes difficult to form an appropriate napping state. Therefore, the toner behaves as an aggregate upon development onto a latent image-bearing member. As a result, problems are apt to occur, which include image quality deterioration such as tailing, image quality deterioration in which an image is collapsed upon fixation owing to an increase in amount of toner mounted on a latent image, and an increase in consumption of the toner.
To cope with those problems, attempts have been made to control the dielectric property of magnetic toner for the purpose of improving the developability of the toner. For example, there has been known toner having a magnetic body with its dispersibility improved by adjusting a dielectric loss tangent (see, for example, Patent Document 1). In this case, however, the stabilization of charging property involved in an environmental fluctuation or in a change with time is not sufficient, for example, when the particle size of the toner is reduced or the amount of the magnetic body is reduced, although the toner has an effect on the stabilization of charging property in a certain environment.
There has been known a technique involving specifying a ratio between a dielectric loss tangent in a high-temperature region and a dielectric loss tangent in a low-temperature region to reduce a change in chargeability of toner due to an environment (see, for example, Patent Document 2). In this case, however, the environmental stability of magnetic toner and a reduction in consumption of the toner are not sufficient.
A one-component development system involves causing toner to pass through a gap between a developing sleeve and a regulating member so that the toner is charged. At this time, a large stress is applied to the toner, with the result that a problem, that is, so-called toner deterioration occurs. In the toner deterioration, a treatment agent subsequently externally added to a toner base particle is embedded in the toner base particle or desorbs from the toner base particle, or the toner base particle becomes chipped. If such deterioration proceeds, a charge amount reduces or the generated fine powder sticks to the developing sleeve or the regulating member when the toner is repeatedly used, so an image defect involved in insufficient charging is apt to occur. To prevent such phenomenon, attempts have been made to sphere magnetic toner to enhance surface smoothness, thereby improving the durability of the toner (see, for example, Patent Document 3). However, this method is also susceptible to improvement in terms of stabilization of charging property due to an environmental fluctuation or the like.
Patent Document 1: JP-A-10-221881
Patent Document 2: JP-A-06-118700
Patent Document 3: JP-A-11-295925